Deck 11: Analytical Chemistry

Question 1

A set of supposedly identical solutions of the dye rhodamine 6G were prepared. The concentrations of the solutions were determined using spectrophotometry and were found to be normally distributed with a mean concentration of 12.672 mmol dm^-3 and a standard deviation of 0.012 mmol dm^-3. What proportion of the samples would be expected to have concentrations greater than 12.696 mmol dm^-3?

A) 2.5%
B) 5.0%
C) 16.0%
D) 32.0%

Answer 1

A

Question 2

The limit for the detection of Cl^- ions in a sample of mineral water through gravimetric analysis is quoted as 2.5 ppb by mass. Express this detection limit as a molar concentration of Cl^- ions, given that the density of water is 1.00 g cm^-3.

A) 2.5  10^-9 mol cm^-3
B) 7.1  10^-11 mol cm^-3
C) 1.3  10^-9 mol cm^-3
D) 3.6  10^-11 mol cm^-3

Answer 2

B

Question 3

1.265 g of hydrated magnesium nitrate, Mg(NO₃)₂.6H₂O were dissolved in 100.0 cm³ of water. 10.0 cm³ of this solution were made up with water to 250.0 cm³. Calculate the concentration of Mg²⁺ ions in the resulting solution.

A) 0.01973 mol dm^-3
B) 0.03411 mol dm^-3
C) 0.04933 mol dm^-3
D) 0.08528 mol dm^-3

Answer 3

A

Question 4

A pH-sensitive electrode was calibrated using a series of aqueous standard solutions and the following data for the potential difference for different H⁺ ion concentrations was obtained. By plotting a calibration graph, determine the H⁺ concentration of a solution for which the measured potential difference was 0.761 V.

A) 1.39  10^-3 mol dm^-3
B) 1.78  10^-3 mol dm^-3
C) 1.09  10^-3 mol dm^-3
D) 1.29  10^-3 mol dm^-3

Question 5

A sample of acetamide is added to a mixture of water and octanol at a temperature of 298 K. If the equilibrium concentration of acetamide in the aqueous layer is 65 mmol dm^-3, what is the concentration of the organic layer? The value of the coefficient for the partition of acetamide between octanol and water at this temperature is, expressed as a logarithm, log₁₀K = -1.16.

A) 4.5 mmol dm^-3
B) 75 mmol dm^-3
C) 56 mmol dm^-3
D) 207 mmol dm^-3

Question 6

In the analysis of the products of a synthesis using thin layer chromatography, spots were observed to have risen distances of 21, 34, 38, and 41 mm up the silica-coated plate, with the solvent front rising to 55 mm. In an analysis of one of the starting materials using the same solvent and type of plate, the spot for the pure compound was observed to rise through 38 mm with the solvent front rising through 62 mm. Which of the spots from the product mixture corresponds to the starting material?

A) 21 mm
B) 34 mm
C) 38 mm
D) 41 mm

Question 7

Propane has a retention time of 4.1 minutes and ethane has a retention time of 2.1 minutes in a particular gas chromatogram. Determine the relative retention of propane to ethane.

A) 1.95
B) 0.51
C) 4.10
D) 0.24

Question 8

The molar absorption coefficient for solutions of [Cu(NH₃)₄]²⁺(aq) ions, is 50 mol^-1 dm³ cm^-1 at a wavelength of 600 nm. Determine the fraction of light of this wavelength transmitted through a sample of a solution of concentration 0.058 mol dm^-3 in a cuvette with a path length of 1.00 cm.

A) 5.5%
B) 2.9%
C) 7.7%
D) 0.13%

Question 9

The absorbance of a solution of the amino acid tryptophan, C₁₁H₁₂N₂O₂, of concentration 1.05 g dm^-3 in a cuvette with a path length of 1.0 cm is 10.29 at a wavelength of 240 nm. Determine the molar absorption coefficient at this wavelength.

A) 5.14  10³ dm³ mol^-1 cm^-1
B) 52.9  10^-3 dm³ mol^-1 cm^-1
C) 2.00  10³ dm³ mol^-1 cm^-1
D) 9.8 dm³ mol^-1 cm^-1

Question 10

Flame photometric analysis of a sample of blood plasma resulted in emission at wavelengths of 589 nm and 760 nm. Which metal ions are present in the sample?

A) Fe²⁺, Mg²⁺
B) Na⁺, K⁺
C) Na⁺, Ca²⁺
D) Fe²⁺, K⁺

Question 11

In order to test the suitability of a solvent for extraction purposes, 1.250 g of solute with molar mass 132.1 g/mol was dissolved in 10.00 mL of water. 5.00 mL of toluene was added, and the system allowed to equilibrate. After extraction 0.925 g of solute is recovered in the organic phase. What is the partition coefficient between water and toluene (expressed as a log K)?

A) log K = −1.76
B) log K = 5.69
C) log K = 1.32
D) log K = −0.76

Question 12

Thin layer chromatography is used to measure a reaction progress. Before the reaction begins, the starting materials are observed to rise 24 mm, 31 mm and 39 mm on a plate where the solvent front moves 59 mm. Some reaction mixture is analysed after 1 hour, and three spots are again observed, at 27 mm, 35 mm and 44 mm, on a plate where the solvent front moves 66 mm. What could be deduced from these observations?

A) No reaction has occurred.
B) On the second plate, the spot at 35 mm represents a reactant.
C) The reaction has proceeded with one of the reactants in excess.
D) On the second plate, the spot at 44 mm represents a reactant.

Question 13

In order to select a column for routine gas chromatography analysis of two compounds A and B, several columns are tried. The retention times (in seconds) for each of the columns are given. Which column would you select?

A) Column 1: A: 16 seconds, B 19 seconds.
B) Column 2: A: 39 seconds, B 54 seconds.
C) Column 3: A: 99 seconds, B 134 seconds.
D) Column 4: A: 34 seconds, B 62 seconds.

Question 14

The absorbance at a particular wavelength was measured for different concentrations of a solution of compound A in a 1.00 cm cell, as shown below. Using a Beer-Lambert plot, determine the molar absorption coefficient.

A) 350 m² mol⁻¹
B) 1229 m² mol⁻¹
C) 8.0 × 10⁻⁶ m² mol⁻¹
D) 1225 m² mol⁻¹

Question 15

A Beer-Lambert analysis showed the extinction coefficient for compound B in water to be 345 m² mol⁻¹ at 450 nm. A different solution of compound B is analysed by UV/visible spectroscopy and the absorbance at 450 nm is recorded as 0.28 in a 1.00 cm cell. What is the concentration of this solution?

A) 8.1 × 10⁻⁴ mol dm^-3
B) 1.2 mmol dm^-3
C) 12 mol dm^-3
D) 8.1 mol dm^-3

Question 16

Two peaks in a particular gas chromatogram have a retention ratio of 1.25. If this mixture was injected into a gas chromatograph column which had the same stationary phase but a longer column, how would you expect the retention ratio to change?

A) It would get larger
B) It would get smaller
C) It wouldn't change much
D) It is not possible to predict

Question 17

A sample of diethyl ether is added to a mixture of water and octanol at a temperature of 293 K. If the equilibrium concentration of diethyl ether in the aqueous layer is 21 mmol dm^-3, what is the concentration of the organic layer? The value of the coefficient for the partition of diethyl ether between octanol and water at this temperature is, expressed as a logarithm, log₁₀K = 0.83.

A) 3.1 mmol dm⁻³
B) 2.1 mol dm⁻³
C) 1.8 mmol dm⁻³
D) 0.14 mol dm⁻³

Question 18

Determine the relative retention time for the two enantiomers shown in Box 11.5

A) 0.60
B) 4.80
C) 1.66
D) 2.89

Question 19

The molar absorption coefficient for solutions of guanosine is 8400 mol^-1 dm³ cm^-1 at a wavelength of 275 nm. Determine the fraction of light of this wavelength transmitted through a sample of a solution of concentration 25 mol dm^-3 in a cuvette with a path length of 1.00 cm.

A) 7.8 %
B) 62 %
C) 11.2 %
D) 1.6 %

Question 20

The concentration of chloride ions in a water sample is 250 ppm. Calculate this concentration in mol dm⁻³.

A) 8.9 M
B) 7.1 mM
C) 4.2 mM
D) 1.4 M

Question 21

A standard solution of potassium chloride of concentration 12.5 mg dm⁻³ was diluted to prepare the solutions shown. Each solution was injected into an atomic emission spectrometer and the emission intensity as shown in the table.

A) 600 ppm
B) 167 ppm
C) 610 ppb
D) 320 ppm

Question 22

The pH of water at 298 K and 1 atm pressure is 7.0. The pH of water
Was measured under these conditions by salespersons for several
Commercial pH meters. Which pH meter would you buy?

A) Meter 1: pH = 7  1
B) Meter 2: pH = 6.90  0.01
C) Meter 3: pH = 7.002  0.004
D) Meter 4: pH = 6.9998  1.0217

Question 23

The pF of a sample is recorded as 3.5. What is the fluoride ion concentration?

A) 6.0 ppm
B) 3.5 ppm
C) 16.6 ppm
D) 1.6 ppm

Question 24

The relative retention for two compounds A and B is quoted as 2.5. From this we can conclude:

A) A will elute from the column first
B) B will elute from the column first
C) A and B will elute from the column at the same time
D) We need more information to draw a conclusion